eB72-1665 is generated against a portion of human toll-like receptor 9 (aa 273-288), a molecule reported to be expressed predominantly intracellularly. TLR9 is a &bsim;115-120kDa molecule, which mediates response to unmethylated CpG dinucleotides in bacterial DNA. CpG DNA induces a strong T-helper-1-like inflammatory response and the proliferation of TLR9-positive human B cells. When stimulated with CpG DNA, TLR9-deficient (TLR9-/-) mice lacked splenocyte proliferation, inflammatory cytokine production from macrophages, and dendritic cell maturation, as compared with normal mice. To date, at least twelve members of the Toll family have been identified. This family of type I transmembrane proteins is characterized by an extracellular domain with leucine-rich repeats and a cytoplasmic domain with homology to the type I IL-1 receptor. Members of the TLR family are involved in recognition and response to different microbial components including lipoproteins, peptidoglycans, and nucleic acids and play important roles in innate immunity and inflammation. TLR9 is not detected by flow cytometry using this antibody on lysed whole human blood and/or isolated human PBMC stained for cell surface or intracellular TLR9. This may be due to limitations of antigen detection by flow cytometry. Human pDCs matured in the presence of IL-3 have been reported to stain with eB72-1665 by immunofluorescence microscopy (Nat Immunol. 5:190). Human Epithelial Cell lines were also reported to stain with this mAb (J. Immunol. 173: 1219). Further studies are needed to determine the relationship between mRNA expression and protein detection by flow cytometry.